User blog:Samuli.seppanen/Correcting bolt flight
In my previous blog post I documented my worst misfire session ever. The fact is that those misfires took me by surprise: before I could pretty much count on the fact that if the arms rotate at the same pace, the bolt will fly straight. But that was with linearly tapering arms, which are way more easy to make in matching pairs than elliptically tillered ones. As mentioned earlier, my plan was to add weight to the faster-moving arm to slow it down and thus correct the bolt flight. And indeed that worked. Below I will outline the process for the benefit of others - the same approach can be used for bows and crossbows as well. However, getting the bolt to leave the weapon at 45 degree angle is difficult without a ballista :). Anyways, I ended up hanging a piece of low-density fiberboard about 12mm thick in front of the backstop: The fiberboard (unlike cardboard) gives some resistance to the bolt, especially when it flies through it slightly tilted, but not too much to break it. So it was really good material for this particular purpose. The first shot showed that the bolt flew with butt-end tilted upwards: After two repetitions it was clear that the problem was not going to go away magically. After a bit of debugging the problem became obvious: the butt-end of the cone was asymmetrical: This made the rest position of the arm to be off-center: This, in turn caused the arm to strike at an upward angle. After each shot the bowstring had rised up considerably from the slider: The problem went away after a bit of rasping and filing and by moving the right arm to the left bundle and vice-versa. The logic behind switching the bundles was to actually make what was a problem (=arm striking upwards) into a feature (=arm striking slightly downwards to ensure it does not rise from the slider). In fact, I think that feature would be quite useful, especially with small bolts which can't have very high butts. The next step was to fix the sideways tilting. Initially it was pretty bad: I initially added too much iron wire to the hook of the faster arm, which actually made the bolt fly (rather humorously) to the backstop butt-end first: After wrapping the correct amount, slightly less than a gram, of iron wire to the hook in the heavier (previously right, now left) arm, some of the shots started going through the chipboard in perfectly straight angle: Yet other shots left the ballista at an angle without any obvious reason. Even flipping the bolt upside down did not produce consistent results. The problem ended up being the bolt itself: it was very short and the head was not in exactly straight line with the shaft. At this point the fiberboard was starting to get into a really bad shape: In the next shooting session I made a new chronograph bolt that weighs about 43 grams, i.e. roughly the same as the one I used for a long while. It has a simple blunt head made from steel pipe, which is fine for this use. Plus it is dead easy to make sure that the head is at the correct angle with the shaft: Here is the earlier short bolt with the new blunt bolt: I then reversed the fiberboard and shot four times through it with the new bolt: all shots went straight through with no problems whatsoever: I assume that the fiber alignment in the fiberboard causes the holes - especially ones created by a blunt bolt - to be wider than they're higher. Before the next session I will make one even heavier bolt and start chronographing the cheiroballistra systematically, ramping up power slowly. I don't see anything stopping me from improving on my previous results. Category:Blog posts Category:Backup Category:Practical